The present invention relates to semiconductor substrates, and more particularly to a semiconductor substrate fabricated to include hermetically-sealed electronic circuitry as well as non-hermetically-sealed electrodes thereon so as to form an implantable sensor or other implantable electronic device.
In U.S. Pat. No. 5,660,163, there is disclosed an implantable glucose sensor which is fabricated on a ceramic substrate. Working electrodes and other elements associated with the sensor are exposed to a conductive fluid contained within a reservoir or inner sheath that covers the substrate. An outer sheath is also placed over the sensor, with a window formed over one of the working electrodes. A selected enzyme, such as glucose oxidate (GO), is placed within the window. As disclosed in the ""163 patent, five wires or conductors are attached to the electrodes and connected to electronic circuitry, e.g., a circuit such as is shown in FIG. 3 of U.S. Pat. No. 5,660,163. U.S. Pat. No. 5,660,163 is incorporated herein by reference.
Additional features, aspects and improvements of a glucose sensor of the type disclosed in U.S. Pat. No. 5,660,163 (hereafter xe2x80x9cthe ""163 patentxe2x80x9d) are further disclosed in U.S. Pat. Nos. 5,999,848; 6,119,028; and 6,081,736; all of which are assigned to the same assignee as the present application, and each of which above-referenced patents is incorporated herein by reference.
As disclosed in the referenced patent applications, an improved implantable sensor may be fabricated by placing the electrodes on one side of the substrate, and by also placing an integrated circuit (IC) chip on the other side of the substrate, along with other needed electronic components, e.g., a capacitor(s), thereby forming a hybrid electronic circuit on the side of the substrate opposite the electrodes that is used to control or drive the sensor (i.e., sense the electrical current flowing to the electrodes, from which current the amount of oxygen near the electrodes can be determined, from which determination, the amount of glucose to which the sensor is exposed can also be determined), as well as to send and receive information, data, and/or power from an external location over a two-conductor transmission line. The IC chip and other electronic components are hermetically sealed under a metal cover, the edges of which are hermetically bonded to the substrate. Electrical connection is established with the IC chip and other sealed components through stair-step vias or passageways that traverse through the substrate. Several of these types of sensors may be daisy-chained together, using just two conductors, as required. The outer sheath encircles the entire substrate, both the electronic circuit side with its metal cover, and the sensor electrode side, with its electrodes, saline solution reservoir and enzyme-filled window.
Disadvantageously, the sensor described in the referenced patent and patent applications is relatively thick. For many implantable applications, a thinner sensor is needed. Hence, there remains a need for yet a smaller sensor that performs all of the same functions as the prior sensor, i.e., that provides working electrodes exposed to a saline solution, with a selected enzyme placed over one electrode, and with hermetically-sealed electronic circuitry controlling the sensor and communicating with other sensors and an external control unit. The present invention advantageously addresses these and other needs.
The present invention provides an implantable substrate sensor wherein electronic circuitry associated with the sensor, i.e., the IC chip, is formed within, or on, a suitable substrate, e.g., a CMOS substrate. A protective coating then covers the substrate, effectively hermetically sealing the circuitry under the coating. Electrodes associated with the sensor are selectively left uncovered by the protective coating, thereby allowing such electrodes to be exposed to body tissue and fluids when the sensor is implanted in living tissue.
Unlike the hybrid sensors of the type described in the referenced patent applications, which include sensor electrodes exposed to enzymes on one side of a substrate, and hybrid electronic circuitry, including an integrated circuit (IC) chip, sealed under a hermetically sealed lid or cover on the other side of the substrate, with stair-stepped vias passing through the substrate to make electrical connection between the electrodes and the hybrid circuitry, the present invention uses the xe2x80x9cchipxe2x80x9d of an IC chip (which contains desired electronic circuitry) as the substrate for the sensor, with the substrate being covered, as required, with a protective coating, and with electrodes being formed on the side of the substrate opposite the electronic circuitry. Such approach advantageously eliminates the need for an hermetically sealed lid or cover on one side of the substrate, and thus allows the sensor to be made much thinner than has heretofore been possible.
The present invention thus takes advantage of the fact that an implantable sensor which includes active electronic circuitry, e.g., an IC chip, which circuitry is already formed on a semiconductor substrate so as to have an active side (where the active circuitry is formed) and an xe2x80x9cnon-active sidexe2x80x9d opposite the active side, may utilize the semiconductor substrate on which the electronic circuitry is formed, i.e., the xe2x80x9cchipxe2x80x9d of the IC, as the substrate for the sensor as well. In particular, for an enzyme-based sensor of the type described in the referenced patent and patent applications, the present invention uses the non-active side of a semiconductor substrate to form the electrodes, and then connects the electrodes on one side of the substrate with the electronic circuitry formed on the active side of the substrate with stair-stepped vias that pass through the substrate. The active side of the substrate, as well as all but the electrode portions of the non-active side of the substrate, are then coated with a coating that hermetically seals the circuitry and allows it to be implanted in living tissue.
In accordance with one aspect of the invention, two thin substrate sensors made in accordance with the present invention, each having electronic circuitry formed on one side of the substrate (and which is covered with a protective covering, as required), with electrodes on the other side of the substrate, may be placed back to back, with the electrodes facing outward. Such back-to-back substrate sensor advantageously allows the sensor electrodes to be positioned of both sides of the substrate, in a package that is no thicker than, and is usually less than, the thickness of the sensors having electrodes only on one side of the substrate.
In accordance with another aspect of the invention, built-in capacitors used by the integrated electronic circuit(s) which is/are formed within the substrate, may be realized by metalization layers and a dielectric that fills the surface area of the substrate anywhere where circuitry and electrodes are not present.
Yet an additional aspect of the invention relates to covering the sensor, except for areas of the electrodes which are deliberately left open to be exposed, with a biocompatible encapsulation material. Such biocompatible encapsulation material provides a protective coating for the sensor, allowing it to be implanted within living tissue or other hostile environments. Such biocompatible encapsulation material is preferably alumina, zirconia, or alloys of alumina and/or zirconia.
The exposed electrodes must, of course, also be made from a biocompatible material. To this end, the electrodes are plated with a biocompatible metal, such as platinum or iridium or alloys of platinum and/or iridium.
A sensor made in accordance with the invention may thus be characterized as a substrate sensor suitable for immersion or implantation in a saline solution, such as within living tissue. Such substrate sensor includes: (a) a semiconductor substrate; (b) at least one electrode formed on one side of the semiconductor substrate; (c) electronic circuitry formed on an opposing side of the semiconductor substrate; (d) a non-straight via that passes through the semiconductor substrate and electrically connects the electrode with the electronic circuitry; (e) a coating that covers the semiconductor substrate with a biocompatible protective layer except for an exposed area of the electrode; and (f) means for coupling operating signals with the electronic circuitry.
The invention may further be characterized as a method of making an implantable substrate sensor, where the sensor includes a substrate, electrodes formed on one side of the substrate, and electrical circuitry formed on the other side of the substrate. Such method includes the steps of: (a) forming the electrical circuitry on an active surface of a semiconductor substrate; (b) forming electrodes on a non-active surface of the semiconductor substrate; (c) electrically interconnecting the electrodes with the electrical circuitry through the use of conductive vias that pass through the body of the semiconductor substrate; and (d) depositing a protective layer of a biocompatible material over the entire surface area of the substrate except for an exposed area of the electrodes so that all but the exposed area of the electrodes is sealed and protected.
It is thus a feature of the invention to provide an implantable sensor having electrodes and electronic circuitry, where the electronic circuitry and electrodes are formed in the same substrate material, e.g., a semiconductor substrate of the same type used in the formation of complementary metal oxide semiconductor (CMOS) integrated circuits.
It is a further feature of the invention to provide an implantable sensor, including electrodes and electronic circuitry, that does not require a lid or cover for hermetically sealing hybrid electronic circuitry on one side of a substrate; thereby allowing the sensor to be significantly thinner than would otherwise be possible.
It is another feature of the invention, in accordance with one embodiment thereof, to provide a thin implantable sensor assembly having electrodes on both sides thereof.